Various approaches of therapeutic vaccination for the treatment of HIV type 1 infection

HIV-1 infection is a major pandemic situation. With the advent of highly active antiretroviral therapy (HAART), morbidity and mortality associated with HIV-1 infection have been dramatically reduced. However, HAART does not enable eradication of the virus. The efficacy of these new regimens is limited by problems over long-term use such as toxicity and resistance. Therapeutic vaccination is an alternative approach to HIV-1 infection. The main aim is to boost and reinforce virus-specific host immune responses. Several immunogens and schedules of immunization have been tested. In this review, various strategies designed for therapeutic vaccines for HIV-1 infection are presented.     

Maternal malaria and parasite adhesion

 Malaria during pregnancy continues to be a major health problem in endemic countries, with clinical consequences, including death, for both mother and child. Just as cerebral malaria results from parasite sequestration in the brain, maternal malaria results from parasite sequestration in the placenta, and a distinct subpopulation of parasites which bind chondroitin sulfate A but not CD36 causes the syndrome. Women have little or no immunological experience with this parasite prior to first pregnancy, making primigravid women particularly vulnerable to infection. Parasites adhere to the surface of trophoblastic villi, eliciting the accumulation of inflammatory leukocytes in the intervillous space, and the necrosis of adjacent placental tissue. Maternal malaria results in poor pregnancy outcomes, although the responsible mechanisms have not been defined. In holoendemic areas both placental infection and poor outcome decrease in frequency with successive pregnancies; protection may result from control of parasite adhesion, suggesting an attractive target for new therapies. Received: 8 April 1997 / Accepted: 19 June 1997     

Proteobacteria-like ferrochelatase in the malaria parasite

A gene encoding the heme biosynthetic enzyme ferrochelatase (FC) was found in the genomic DNA databases of Plasmodium spp. The predicted amino acid sequence of malarial FC is highly conserved and fairly well conserved by comparison with other orthologues. The FC genes of P. falciparum and P. yoelii are transcribed and the mRNAs are processed to encode polypeptides of the expected amino acid sequence. The cloned cDNA for the FC of P. falciparum successfully rescued a FC-null mutant of Escherichia coli, indicating that it encodes an active enzyme. Unlike eukaryotic FCs, the malarial enzyme lacks a characteristic extension at the C-terminus. In addition, the sequence of the malarial FC resembles proteobacterial orthologues rather than eukaryotic enzymes. Strikingly, the malarial FC lacks a bipartite presequence at its N-terminus, unlike delta-aminolevulinic acid dehydratase of the same organism. This suggests an unusual intracellular distribution of heme biosynthetic enzymes, involving multiple subcellular compartments.     

Whole parasite vaccines for the asexual blood stages of Plasmodium

After many decades of research, an effective vaccine for malaria is still not available. Most research efforts have focused on identifying a key target antigen and then using powerful adjuvants to generate specific antibodies that can block parasites from entering host cells (hepatocytes, red blood cells). However, the inability to generate sufficiently potent antibody responses has led to significant disappointment with current vaccine programs. An additional challenge for sub-unit vaccines is that key vaccine antigens are highly polymorphic. These challenges have spurred radically different approaches to malaria vaccine development. Many of these involve the use of “whole parasites”—either extracted from mosquitoes or cultured. With these, every parasite molecule for that particular strain is included in the vaccine. This strategy is showing great promise following several clinical trials with irradiated sporozoites. However, a whole-parasite approach to a blood stage vaccine has not advanced as quickly. This article outlines the history, the different approaches that are being taken and the challenges associated with whole parasite blood stage vaccines and discusses recent exciting developments as these vaccines now move into the clinic.     

Erythrocyte surface glycosylphosphatidyl inositol anchored receptor for the malaria parasite

Parasitophorous vacuole formation is a critical step for the successful invasion of host erythrocytes by the malaria parasite. Rhoptry proteins are believed to have essential roles in vacuole formation, although their biological roles are poorly understood. To understand the molecular interactions between parasite rhoptry proteins and the erythrocyte during invasion, we have characterized the binding specificity of the high molecular mass rhoptry protein (RhopH) complex to erythrocytes using the rodent malaria parasite, Plasmodium yoelii. RhopH complex binding to erythrocytes was species-specific, observed with mouse but not rabbit or human erythrocytes. Binding is abolished following treatment of erythrocytes with trypsin or chymotrypsin. Because host cell cholesterol-rich membrane domains are recruited into the nascent parasitophorous vacuole, we evaluated a possible role of RhopH complex binding to the cholesterol-rich membrane domain-associated glycosylphosphatidyl inositol (GPI)-anchored protein. Using chimeric mice harboring GPI-deficient erythrocytes, RhopH complex binding to GPI-deficient mouse erythrocytes was undetectable, indicating involvement of GPI-anchored protein in PyRhopH complex binding. Furthermore, a significant reduction of P. yoelii parasite infection of GPI-deficient erythrocytes was observed in vivo, probably due to inefficient invasion. We conclude that the major erythrocyte receptor for PyRhopH complex is a protein attached to the erythrocyte surface via GPI-anchor and that GPI-deficient erythrocytes are resistant to P. yoelii invasion.     

Toll-like receptor激动剂耐受分子机制和疟原虫慢性感染

小剂量LPS预刺激可引起内毒素耐受,能提高小鼠在致死剂量IPS再次刺激后的存活率,其机制可能与铎样受体TLR(Toll-like receptor)4信号通路的下调有关.近年来的研究发现,其他TLR激动剂同样可以引起宿主对相应TLR激动剂和LPS的耐受,后者称为交叉耐受(cross tolerance),而且疟原虫慢性感染同样能诱导类似TLR激动剂耐受的现象,因此,研究TLR激动剂耐受分子机制有助于脓毒血症预防和对慢性感染机制的理解.     

Hepatitis B virus infection, its sequelae, and prevention by vaccination

Hepatitis B virus (HBV) infection is a global health problem. There are >350 million of people chronically infected with this virus worldwide. Hepatitis B vaccines are effective in preventing the infection. Humoral immunity is the key factor in conferring the protection. Hepatitis B surface antibody titers of ≥10mIU/mL are protective. Chronic carriage of HBV is related to the age when the infection occurs, the younger the age the higher the chronicity rate. Hence, vaccination should be given in early childhood. People vaccinated in infancy have a protection of >20 years, and hepatocellular carcinoma decreases in them. Although the vaccine-conferred immunity wanes by time, a universal booster is not recommended at present.     

Antibody and T-cell responses associated with experimental human malaria infection or vaccination show limited relationships

This study examined specific antibody and T-cell responses associated with experimental malaria infection or malaria vaccination, in malaria-naive human volunteers within phase I/IIa vaccine trials, with a view to investigating inter-relationships between these types of response. Malaria infection was via five bites of Plasmodium falciparum-infected mosquitoes, with individuals reaching patent infection by 11–12 days, having harboured four or five blood-stage cycles before drug clearance. Infection elicited a robust antibody response against merozoite surface protein-1₁₉, correlating with parasite load. Classical class switching was seen from an early IgM to an IgG1-dominant response of increasing affinity. Malaria-specific T-cell responses were detected in the form of interferon-γ and interleukin-4 (IL-4) ELIspot, but their magnitude did not correlate with the magnitude of antibody or its avidity, or with parasite load. Different individuals who were immunized with a virosome vaccine comprising influenza antigens combined with P. falciparum antigens, demonstrated pre-existing interferon-γ, IL-2 and IL-5 ELIspot responses against the influenza antigens, and showed boosting of anti-influenza T-cell responses only for IL-5. The large IgG1-dominated anti-parasite responses showed limited correlation with T-cell responses for magnitude or avidity, both parameters being only negatively correlated for IL-5 secretion versus anti-apical membrane antigen-1 antibody titres. Overall, these findings suggest that cognate T-cell responses across a range of magnitudes contribute towards driving potentially effective antibody responses in infection-induced and vaccine-induced immunity against malaria, and their existence during immunization is beneficial, but magnitudes are mostly not inter-related.     

Development of a multiplex PCR-ligase detection reaction assay for diagnosis of infection by the four parasite species causing malaria in humans

The diagnosis of infections caused by Plasmodium species is critical for understanding the nature of malarial disease, treatment efficacy, malaria control, and public health. The demands of field-based epidemiological studies of malaria will require faster and more sensitive diagnostic methods as new antimalarial drugs and vaccines are explored. We have developed a multiplex PCR-ligase detection reaction (LDR) assay that allows the simultaneous diagnosis of infection by all four parasite species causing malaria in humans. This assay exhibits sensitivity and specificity equal to those of other PCR-based assays, identifying all four human malaria parasite species at levels of parasitemias equal to 1 parasitized erythrocyte/microl of blood. The multiplex PCR-LDR assay goes beyond other PCR-based assays by reducing technical procedures and by detecting intraindividual differences in species-specific levels of parasitemia. Application of the multiplex PCR-LDR assay will provide the sensitivity and specificity expected of PCR-based diagnostic assays and will contribute new insight regarding relationships between the human malaria parasite species and the human host in future epidemiological studies.     

Merozoite vaccination against Plasmodium knowlesi malaria.

Free malarial merozoites isolated from in vitro cultures of P. knowlesi and emulsified with Freund's complete (FCA) or incomplete (FIA) adjuvant were used to vaccinate twelve Rhesus monkeys against the uniformly lethal infection caused by P. knowlesi. Initial challenge of six monkeys with the same parasite variant as used for vaccination produced no detectable infection in three monkeys, while three others developed low-grade parasitaemia (maximum 1.5 per cent), which terminated after 6-11 days. Vaccination with merozoites in either FCA or FIA induced protection against homologous variant challenge. Six other monkeys were challenged first with a parasite variant different from that used for vaccination. Two animals immunized with merozoites in FIA alone or in FCA on only one occasion developed fatal infections. The other four animals vaccinated at least twice with merozoites in FCA showed low-grade parasitaemia (maximum 1.5 per cent) which terminated after 8-12 days. Eight monkeys rechallenged on eleven occasions at intervals of up to 16 weeks were completely resistant to several variants and a distinct laboratory strain of P. knowlesi, but developed chronic malaria similar to that in unimmunized controls when challenged with a different species of malaria, P. cynomolgi bastianellii. It is concluded that merozoite vaccination of Rhesus monkeys induces immunity against the erythrocyte stages of P. knowlesi far greater in degree and significantly broader in variant specificity than that achieved by previous methods of immunization or by repeated drug-controlled infections.     

A malaria parasite toxin associated with Plasmodium vivax paroxysms

We have previously demonstrated a correlation between clinical paroxysms in Plasmodium vivax malarial infections and the appearance in patients' plasma of factors that kill blood stage parasites (gametocytes). This activity was, as previously shown, dependent on the presence in paroxysm plasma of tumour necrosis factor-alpha (TNF-α), which acts in conjunction with other ‘complementary' factors. Here we have identified a parasite component which is essential for this activity and functions as a ‘complementary' factor together with TNF, and a third component of unknown origin. The P. vivax parasite component present in paroxysm plasma can be substituted for by a blood-stage schizont extract of either P. vivax or P. falciparum. This was demonstrated by restoring the parasite-killing activity to post-paroxysm plasma (from which it was absent) with the addition of the extracts together with TNF. The active materials in these extracts, however, are different from the natural components in P. vivax paroxysm plasma, i.e. while the schizont extracts are immunologically cross-reactive between species, the activity of the natural P. vivax toxin(s) in patients' plasma is neutralized only by the homologous antisera. Plasmodium falciparum infections have neither distinct paroxysms nor parasite-killing activity in plasma. The pronounced paroxysms of P. vivax infections may thus be due in part to a species-specific toxin(s).     

Osteopontin participates in Th1-mediated host resistance against nonlethal malaria parasite Plasmodium chabaudi chabaudi infection in mice

Osteopontin (OPN) knockout mice (OPN-KO mice) died of Plasmodium chabaudi chabaudi infection, although wild-type (WT) mice had self-limiting infections. OPN was detected in the WT mice at 2 days postinfection. OPN-KO mice produced significantly smaller amounts of interleukin-12 and gamma interferon than WT mice produced. These results suggested that OPN is involved in Th1-mediated immunity against malaria infection.     

Mechanisms of transfusion-linked parasite infection.

Besides bacteriae and viruses, parasite infections can be transmitted by blood as a consequence of blood transfusion. To be transmitted by blood transfusion, parasites must: -i) circulate in the blood stream of donors, -ii) comprise of certain physical characteristics and resist processing steps leading to the preparation of labile blood products (packed red blood cells, therapeutic frozen plasma, or platelet concentrates), -iii) survive conservation; further, to generate infection in the blood receiver, such parasites must retain infectivity. In this Forum, the main characteristics of parasites-with respect to transmissibility by blood exposure-are recalled. This forum will further discuss each of these topics and try to make an issue with possible risks in transfusion, in countries that are either non-endemic for the given parasite or which can afford implementation of preventive measures.